1. Field of the Invention
This invention relates to CD-ROM (compact disc read-only memory) technology, and more particularly, to a programmable frequency following device which can be utilized on a CD-ROM drive with an operating frequency range of from 33 MHz to 66 MHz (megahertz), and which is capable of being programmed to any desired frequency in the 33-66 MHz range through software/firmware setting and microprocessor control without having to replace the associated external oscillator that is used to generate the system clock signal.
2. Description of Related Art
A CD-ROM drive is a widely used mass data storage device for a personal computer (PC). During read operation, the data read from the CD-ROM are first decoded through a digital signal processor (DSP) and then transferred to the RAM of the PC for processing. The performance of a CD-ROM drive is dependent on the frequency of the clock signal used to drive the CD-ROM drive--the higher the clock frequency, the higher is the performance of the CD-ROM drive. Therefore, the stability of the clock generator for the CD-ROM drive is a primary concern in CD-ROM technology.
Conventionally, a CD-ROM drive uses an oscillator to generate a clock signal which is then either directly used to drive the CD-ROM drive or multiplied in frequency before being used. The frequency multiplication is conventionally achieved by using a phase-locked loop (PLL) circuit. Beside frequency multiplication, there are some new circuits that can be used to generate a high-frequency clock signal.
These conventional clock generators, however, are mostly based on analog circuitry. One drawback to the use of analog clock generators is that they are complex in circuit structure and would be low in stability. Moreover, since today's information processing systems are largely based on digital circuitry, an analog circuit would be less easily to be integrated to a digital system. Therefore, there exits a need for a programmable frequency following device that is based on digital circuitry so that it would be easily integrated to any digital systems.